Microwave phase shifter



Aug- 16, 1965 H. H. KEELING ETAL 3,267,395

MICROWAVE PHASE SHIFTER 2 Sheets-Sheet 1 Filed Oct. 31, 1961 Aug' 16, 1966 H. H. KEELING ETAL 3,267,395

MICROWAVE PHASE SHIFTER 2 Sheets-neet 2 Filed Oct. 31. 1961 United States Patent O 3,267,395 MICROWAVE PHASE SHIFTER Harmon H. Keeling, Los Angeles, and Thomas Hudspeth, Malibu, Calif., assiguors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed (Ict. 31, 1961. Ser. No. 151,135 3 Claims. (Cl. S33- 31) The present invention relates to microwave phase shifters and, more particularly, to a device providing a phase shift variable over a wide range in the microwave region in response to an electrical control signal.

Phase Shifters nd wide utility in the electronic arts for such purposes as aiming the radiation pattern of an antenna array, for example. At microwave frequencies, variable phase Shifters usually take the form of mechanically varied devices such as mechanically adjustable trombone transmission lines or, alternatively, transmission lines having mechanically adjustable short circuits and being utilized in conjunction with a hybrid junction. It is often desirable that a variable phase shifter be electrically, rather than mechanically, controlled for ease of automatic operation, as in a scanning antenna array.

Electrically variable phase Shifters Afor use in the microwave region may employ a resonant circuit containing an impedance element having an electrically varied reactance such as a voltage-variable capacitor-diode or an inductance element containing ferrite and utilizing a variable polarizing magnetic field to vary the inductance thereof. When a variable inductance element is used in a microwave phase shifter, it is diflicult to obtain a sufficiently large phase change, such as from -l80 for example, because this Anecessitates a large percentage change in the inductance element or, alternatively, impractically low inductance in the resonant circuit. When ferrite is used to vary the inductauce, ineicient regions of operation inherent in ferrite, and at which power losses are extremely high, are often reached as the magnetic lield is increased, before suiiicient phase shift is obtained. This is particularly true at frequencies in the vicinity of 2 kilomegacycles per second at which operation is often desired.

Accordingly, it is an object of the present invention to provide an improved phase shifter that produces an electrically controllable phase shift from 0-l80 -at microwave frequencies in the vicinity of 2 kilomegacycles per second.

In accordance with this and other objects of the invention, there is provided a directional coupler having an input terminal, an output terminal, and two intermediate terminals. Each of the intermediate terminals is connected by a 1A; wavelength section of high impedance transmission line to a Mt wavelength section of low impedance transmission line having a short circuit at the -far end. Ferrite elements are disposed in each of the 1A wavelength sections of transmission line adjacent the short circuit. A magnetic ield is passed through the ferrite elements in a direction perpendicular to the magnetic eld of waves propagated in the transmission lines. By varying the strength of the magnetic field passing through the ferrite elements, the eifect of changing the electrical distance to the short circuits is produced. The 1/s wavelength sections of high impedance transmission line inserted between the directional coupler and the short circuited sections of low impedance transmission line enhance or multiply the effect produced by the ferrite elements so that a relatively small change in the magnetic Vfield causes an apparent 1A wavelength change in the electrical distance to the short circuits. The magnitude of this multiplication eifect is proportional to the ratio of the characteristic impedance of the A wavelength seclocation.

3,267,395 Patented August 16, 1966 ICC tion of low impedance transmission line to the characteristic impedance of the lz wavelength section of high impedance transmission line. In this manner, 0-180" phase change between the input and output terminals of the directional coupler is obtained without increasing the magnetic field to the point where the .ferrite elements are operating in an ineicient region of high power loss.

The following specification and the accompanying drawings respectively describe and illustrate an exemplication of the present invention. Consideration of the specification and the drawings will provide a complete understanding of the invention, including the novel Ifeatures and objects thereof. Like reference characters designate like parts throughout the figures of the drawings.

FIG. l is a side elevation View of the exterior of an embodiment of an electrically variable transmission line assembly constructed in accordance with the present invention and for use with a directional coupler as a variable phase shifter;

FIG. 2 is an exterior elevation view of the short circuit end of the electrically variable transmission line assembly of FIG. l;

FIG. 3 is an enlarged plan View of the interior of the lower portion of the variable transmission line assembly of FIGS. l and 2, taken along the lines 3-3 of FIG. l;

FIG. 4 is a side elevation View in cross section of the lower portion of the variable transmission line assembly of FIG. 3;

FIG. 5 is a perspective view of a portion of the variable transmission line assembly of FIGS. 3 and 4, showing in detail the configuration and arrangement of the short circuit and the ferrite element;

FIG. 6 isa perspective view of a partially disassembled directional coupler suitable for use with the electrically variable transmission line assembly of FIGS. 1-5, and showing the cover removed from and placed alongside the case, with the mating surfaces up; and

FIG. 7 is a perspective view of an embodiment of a microwave phase shifter in accordance with the present invention and employing the electrically variable transmission line assembly of FIGS. 1-5 and the directional coupler of FIG. 6.

Referring now to FIGS. l and 2, there is provided an electrically variable transmission line assembly, indicated generally at 10, and having a pair of identical upper and lower housing members 11 and 12 disposed symmetrically with respect to each other. The housing members 11 and 12 are generally rectangular in configuration and are made of non-magnetic metal such as aluminum. A tlat metal separating -plate 13, also of aluminu-m, is disposed between the two symmetrically disposed housing members 11 and 12 yand the whole assembly 10 is fastened together by screw-s or other suitable means. A coaxial cable connector 14 it attached to the upper housing member 11 on the surface opposite the separating plate 13 and near the end of the housing member 11. Similarly, a coaxial cable connector 15 is also provided on the lower housing member 12 4in the corresponding symmetrical A groove 16 is provided in the surface of the upper housing member 11 opposite the separating plate 13 near the end opposite the connector 14 and is disposed transverse to the longitudinal dimension of the housing member 11. Similarly, a symmetrically disposed groove 17 is provided in the lower housing member 12. A generally C-shaped magnetic core 18 has its ends disposed in the grooves 16 and 17 provided in the housing -members 11 and 12 and has a wire coil 20 wound thereon.

The end of the transmission line assembly 10 adjacent the grooves 16 and 17 in the housing members 11 and 12 -is shown in FIG. 2, the magnetic core 18 being broken away; and it may be seen that the housing members 11 and 12 are provided with elongated narrow openings 21 and 22 parallel with, and adjacent to, the separating plate 13. FIGS. 3 and 4 show the lower housing member 12 with the separating plate 13 removed therefrom to expose the interior thereof. inasmuch as the housing members 11 and 12 are identical in construction and symmetrically arranged, the following description of the interior of the lower housing member 12 may be taken as representative of the arrangement of the interior of the upper housing member 11. A relatively deep, substantially square, depression or hollow chamber 23 is formed in the lower housing member 12, adjacent the end thereof to which the coaxial connector lis fastened. The coaxial connector 15 has a center conductor that extends into the chamber 23 near one end thereof. The narrow opening 22 in the end of the housing member 12 extends in the longitudinal direction to the deep chamber 23 to form a relatively shallow, substantially rectangular, depression or hollow chamber 24, that opens into the deep chamber 23.

A relatively narrow fiat metal strip 25 is disposed within, but spaced away from the walls of, the deep chamber 23 and is connected at one end, `as by soldering, to the center conductor of the coaxial connector 15. The narrow strip 25 is wavelength long at the frequency of operation which, in the present example, is 2 kilomegacycles per second. The planar surface of the narrow strip 25 is oriented transverse to the planar surface of the separating plate 13. This orientation of the narrow strip 25 in the relatively large and deep chamber 23, provides a minimum of capacitance between the narrow strip 25 `and the walls of the chamber 23.

A relatively wide at metal strip 26 that is 1A wavelength long at the frequency of operation is disposed in,

but spaced away from the walls of the shallow chamber 24. The wide strip 26 is oriented with its planar surface parallel to the planar surface of the separating plate 13 and extends from the deep chamber 23 to the opening 22 at the end of the lower housing member 12. The end of the wide strip 26 adjacent the deep chamber 23 is provided with a narrow projection 27 that is bent at rightangles to extend into the deep chamber 23 parallel with the center conductor of the coaxial connector 15. The other end of the narrow strip 25 is connected to the projection 27`extending from the wide strip 26, as by soldering. A first conductive contact 27 made of a resilient metal such as Phosphor bronze, and formed into an elongated configuration having a U-shaped cross section, is disposed between the wide strip 26 and the separating plate 13 in and coextensive with the opening 22. A second conductive contact 28, identical in construction to the first contact 27, is disposed between the wide strip 26 and the housing member 12 in and coextensive with the opening 22. These contacts 27 and 28 are held in place by frictional contact and spring tension inasmuch as they are compressed when in place. The conductive contacts 27 and 28 are movable so that slight adjustments in their location may easily be made and yet they provide low resistance electrical contact between the end of the wide `strip 26 and the housing member 12 and separating plate 13.

A first rectangular ferrite element 30 isdisposed adjacent the lower conductive contact 28, between the wide strip 26 and the housing member 12, and generally coextensive with the groove 17 in the exterior of the housing member 12. A second rectangular ferrite element 31 -is disposed adjacent the upper contact 27, between the wide strip 26 and the separating plate 13, also coextensive with the groove 17 in the housing member 12. First and second spacers 32 and 33 made of an insulating material such as Styrofoam, for example, are disposed on both sides of the wide strip 26, adjacent the ferrite elements 30 and 31 and serve to hold them in place. The `ferrite elements 30 and 31 are, in the present example, of the 4 yttrium-iron-garnet variety and may be of a type manufactured by the Microwave Chemicals Laboratory, Inc. 282 Seventh Ave., New York, New York, identified by the number MCL1116. The characteristics of this material are specified as: a saturation magnetization of 600 gauss, a linewidth (at 4 kilomegacycles) of 50 oersteds, and a Curie temperature of 170 centigrade.

With the separating plate 13 in place over the lower housing member 12,.the chambers 23 and 24 are completely enclosed and form, with the Kstrips 25 and 26, a microwave transmission line. This type of transmission line may be generally categorized as fiat-strip transmission line and is similar to a coaxial transmission line in that the walls of the housing 'member 12 and the separating plate 13 serve as an outer conductor, and the strips 25 and 26 serve as an inner conductor. The coaxial cable connector 15 is the terminal of the device and is connected by a ls wavelength section of high impedance transmission line to a 1A wavelength section of low impedance transmission line having a short circuit at the far end. The narrow strip 25 in the deep chamber 23 forms the la wavelength section of high impedance transmission line. The high impedance results in part from the low capacitance between the strip 25 and the walls of the chamber 23. The characteristic impedance, in the present example, ohms. The wide strip 26 in the shallow chamber 24 forms the 1A: wavelength section of low impedance transmission line, and the characteristic impedance is, in the present example, 20 ohms. The short circuit at the end of the 1A wavelength section of transmission line is formed by the conductive contacts 27 and 28 which electrically connect the side strip 26 to the housing member 12 and t0 the separating plate 13.

The ferrite elements 30 and 31 adjacent the short circuit modify the characteristics of the 1A wavelength section of transmission line. A wave propagated down the transmission line is reiiected from the short circuit and must pass through the ferrite elements 30 and 31 twice. The propagation velocity of the wave through the ferrite is variable and changes the apparent length of the transmission line. The propagation velocity through the fer- -rite depends upon the permeability of the ferrite. The magnetic core 20 and the coil 21 Wound thereon produces, when energized, by a source of potential a magnetic field that passes through the ferrite elements 30 and 31 in a direction perpendicular to the magnetic field of waves propagated in the transmission line. The magnetic field varies the permeability of the ferrite, as the field increases, the permeability decreases and the M1 wavelength section of transmission appears to shorten in length.

Referring now to FIGS. 6 and 7, there is illustrated a directional coupler 40 which, as is well-known, is a form of microwave bridge or hybrid network. The directional coupler 40 utilizes fiat-strip transmission line and is constructed in accordance with the general principles set forth on pages 78-80 of the book Handbook of Tri-Plate Microwave Components, published in 1956 by Sanders Associates, Inc., Nashua, New Hampshire. The directional coupler 40 has a rectangular case 41 made of metal such as aluminum and having a shallow rectangular depression 42 therein. A matching rectangular cover 43, also -made of metal such as aluminum, is provided and is adapted to be fastened to the case 41 over the depression 42 by screws or the like. Four coaxial cable connectors 44-47 are fastened to the cover 43 and have center conductors that extend into the rectangular depression 42 in the case 41 near the corners thereof. A narrow metal strip 48 is fastened to, and interconnects, the center conductors of all four of the connectors 44-47. The strip 48 extends along, but spaced away from, the four side walls of the depression 42 and defines the perimeter of a rectangle having a central opening, the mean length of each side of the rectangle being 1A wavelength. The planar surface of the strip 48 is parallel to the planar surface of the cover` 43. The strip 48 is slightly wider between conlconnector of the electrically variable transmission line Iasse-mbly'ltb by a section of 50 ohm coaxial transmission line 51 ofthe same length.

The following is a simplied explanation for clarity in understanding the operation of the invention. A microwave signal applied at connector 47 of the directional coupler 40 from a microwave source produces two lforward waves in the electrically variable transmission line assembly 10 which are equal in magnitude and 90 out of phase, the wave propagated in the lower housing member 12 lagging behind the other wave. These Waves are reflected from the short circuits and returned to the directional coupler 40 with the wave at connector 45 still lagging behind the wave at connector 44 by 90, both the reflected waves having a lag or phase delay with respect to the forward waves of twice the electrical distance from the connectors 45 and 44 to the short circuits, plus 180 introduced yby the reflection from the short circuits. The reected waves arrive in-phase with each other at connector 46 because there is 90 more lag in traversing the path between connectors 44 and 46 than between connectors 45 and 46. Since there is 90 more lag in coupling from connector 45 to connector 47 than from connector 46 to connector 47, the coupled reflected waves arrive at connector 47 in opposition, producing no net emerging wave at connector 47. The output wave at connector 46 varies in phase with respect to the input wave at connector 47 in accordance with the apparent position of the short circuits in a linear manner. 180 phase change results from an apparent change of 1A wavelength in the position of the short circuits. The apparent position of the short circuits is varied by applying a source of variable electrical potential to the coil 2t) to change the permeability of the ferrite.

For purposes of analysis, the 1A: wavelength section of low impedance transmission line having a short circuit at the far end and ferrite therein may be thought of as a resonant circuit having a small variable inductance element. 'I'he input impedance of the 14s wavelength section of high impedance transmission line varies from zero to infinity capacitive reactance, as shown on a Smith chart, when the susceptance of the `resonant circuit, normalized to the ls wavelength transmission line admittance, varies from +1 through 0 to 1. Changing the input impedance of the 1A; wavelength section of transmission line from 0 to infinity capacitive reactance is equivalent to 1A wavelength motion of the short circuit which produces a phase change of 0 to 180 at the directional coupler 40. That is to say, if mechanically movable short circuits had been provided instead of the arrangement described, and if these short circuits were moved over a distance of 1A wavelength, the linput impedance of the 14s wavelength lines would change from 0 to infinity capacitive reactance and, as a result, a phase change of 0 to 180 would be produced at the directional coupler 40. Hence, anything which produces a change from 0 to infinity capacitive reactance at the input of the 1/8 wavelength sections of transmission line will produce a phase change of 0 to 180 at the directional coupler 40. As stated before, the equivalent resonant circuit produces such a change of input impedance, when normalized to the 1A; wavelength transmission line admittance. Thus, the necessary un-normalized susceptance change of the inductance element to produce a given change at the input of the ls wavelength section of transmission line is reduced by the ratio of the impedance of the 1A wavelength section of transmission line to the impedance of the 1A; Wavelength transmission line. An i-mprovement of several times in the phase shift produced per |unit susceptance change in the inductance element is thus obtained by the insertion of the 14; wavelength section of high impedance transmission. line between'the directional coupler 40 and the resonant circuit. The result is that a phase change from'0 to 180 is obtained without necessitating an extremely large inductance change -in the inductance element. Accordingly, it is not necessary to provide a large change in the permeability of the ferrite to produce a phase change from 0 to 180. In the present embodiment of the invention, it has been found that a change in the magnetic field of approximately 0 to 500 gauss produces a phase change of 180. This magnetic field intensity is on the low field side of the ferromagnetic resonance region of the ferrite material. Hence, the inefficient region of operation of the ferrite, at which power losses are high, is avoided. Furthermore, on the weak eld side of resonance, the change of permeability per unit change of magnetic flux is greater.

Thus, there has been described a phase shifter that produces an electrically controllable phase shift over a range of from 0 to 180 at microwave frequencies, and which is operable in the region of 2 kilomegacycles per second. The phase shifter of the present invention utilizes magnetically va-ried ferrite elements in short circuited 1A wevelength sections off low impedance transmission line coupled to a directional coupler by 1A; wavelength sections of high impedance transmission line.

Having thus described the invention and the present embodiment thereof, it is desired to emphasize the fact that many modifications may be resorted to in a manner limited only Iby a just interpretation of the following claims.

What is claimed is:

1. A microwave phase shifter comprising: a directional coupler having an input terminal to receive an input microwave signal, an output terminal, and a pair of intermediate terminals; a pair of 14; wavelength sections of high impedance transmission line, each having one end individually coupled to a different one of said intermediate terminals; a pair of 1A wavelength sections of low impedance transmission line, each having one end individually coupled to the other end of a different one of said 1A; Wavelength sections of transmission line; a short circuit member disposed at the other end of each of said 1A wavelength sections of transmission line; a pair of ferrite elements, each being disposed within a different one of said 1A wavelength sections of transmission line adjacent said short circuit members; and magnetic field producing means juxtaposed with said 1A w-avelength sections of transmission line and disposed to provide a magnetic field of predetermined intensity -through said ferrite elements in a direction perpendicular to the magnetic eld of a microwave signal propagated in said 1A wavelength sections of transmission line.

2. A microwave device comprising: a directional coupler having an input terminal to receive an input micro- Wave signal, an output terminal, and a pair of intermediate terminals; a pair of high impedance transmission lines substantially 1/s wavelength long, each of said high impedance transmission lines having one end individually coupled to a different one of said intermediate terminals; a pair of low impedance transmission lines substantially 1A: Wavelength long, each of said low impedance transmission lines having one end coupled to the other end of a different one of said high impedance transmission lines; a short circuit member disposed at the other end of each of said low impedance transmission lines; a pair of ferrite elements, each being disposed within a different one of said low impedance transmission lines adjacent said short circuit members; and magnetic eld producing means juxtaposed with said low impedance transmission lines and disposed to provide a magnetic field of predetermined 7 intensity through said ferrite elements in a direction perpendicular to the magnetic eld of a microwave signal propagated in said low impedance transmission lines.

3. An electrically controllable microwave transmission line of the at strip type comprising: a conductive outer case having therein walls delining a shallow chamber opening into a deep chamber; an input terminal member for a microwave signal extending into said deep chamber and insulated therefrom; a at, narrow conductive strip substantially 1A; wavelength long disposed in said deep chamber, spaced away from the walls thereof, and having one end conductively connected to said terminal member; a flat, Wide conductive strip substantially 1A wavelength long disposed in said shallow chamber proximate to the walls thereof and spaced away therefrom, one end of said wide strip being conductively connected to the other end of said narrow strip, the other end of said wide strip being conductively connected to said case to terminate said transmission line in a short circuit; a ferrite element disposed within said case adjacent said wide strip and proximate to said short circuit; and electromagnetic means juxtaposed with said case and disposed to provide a magnetic eld of predetermined intensity through said ferrite element in a direction perpendicular to the magnetic eld of a microwave signal propagated in said transmission line.

References Cited by the Examiner UNITED STATES PATENTS HERMAN KARL SAALBACH, Primary Examiner.

C. BARAFF, Assistant Examiner. 

1. A MICROWAVE PHASE SHIFTER COMPRISING: A DIRECTIONAL COUPLER HAVING AN INPUT TERMINAL TO RECEIVE AN INPUT MICROWAVE SIGNAL, AN OUTPUT TERMINAL AND A PAIR OF INTERMEDIATE TERMINALS; A PAIR OF 1/8 WAVELENGTH SECTIONS OF HIGH IMPEDANCE TRANSMISSION LINE, EACH HAVING ONE END INDIVIDUALLY COUPLED TO A DIFFERENT ONE OF SAID INTERMEDIATE TERMINALS; A PAIR OF 1/4 WAVELENGTH SECTIONS OF LOW IMPEDANCE TRANSMISSION LINE, EACH HAVING ONE END INDIVIDUALLY COUPLED TO THE OTHER END OF A DIFFERENT ONE OF SAID 1/8 WAVELENGTH SECTIONS OF TRANSMISSION LINE; A SHORT CIRCUIT MEMBER DISPOSED AT THE OTHER END OF EACH OF SAID 1/4 WAVELENGTH SECTIONS OF TRANSMISSION LINE; A PAIR OF FERRITE ELEMENTS, EACH BEING DISPOSED WITHIN A DIFFERENT ONE OF SAID 1/4 WAVELENGTH SECTIONS OF TRANSMISSION LINE ADJACENT SAID SHORT CIRCUIT MEMBERS; AND MAGNETIC FIELD PRODUCING MEANS JUXTAPOSED WITH SAID 1/4 WAVELENGTH SECTIONS OF TRANSMISSION LINE AND DISPOSED TO PROVIDE A MAGNETIC FIELD OF PREDETERMINED INTENSITY THROUGH SAID FERRITE ELEMENTS IN A DIRECTION PERPENDICULAR TO THE MAGNETIC FIELD OF A MICROWAVE SIGNAL PROPAGATED IN SAID 1/4 WAVELENGTH SECTIONS OF TRANSMISSION LINE. 